1. Field of the Invention
The present invention relates to a silicon pressure sensor and the manufacturing method thereof, which is enabled with the low-temperature drift and the directionality for forming a layout on a membrane by the silicon-piezoresistor and interconnects. Thus, a maximum pressure-sensing signal can be obtained, and it is correspondent with the simplicity to balance the Wheatstone bridge circuit.
2. Description of the Related Background
The piezoresistive pressure sensor device is one of the pressure-sensing devices for the semiconductors, and the principle thereof is to sense the pressure from the outward environment by means of the piezoresistance effect of the silicon. In 1954, the piezoresistance effect is found by C. S. Smith [1]. A conventional approach has been to arrange four resistors in a Wheatstone bridge configuration. Small changes in the individual resistance values contribute to a significant offset in the bridge and provide an easily detectible signal.
Classical strain gages are made of fine metal wire for sensing the stress. The gage factor of semiconductors is more than an order of magnitude higher than that of metal. Good matching of the resistors can be achieved, which is particularly useful if Wheatstone bridges are used. The technique is very suitable for miniaturization of the sensors. It is able to integrate other circuit directly on the same wafer, for signal amplification and temperature compensation. The piezoresistive pressure sensor has some of the practical advantages.
Piezoresistors are fabricated onto a membrane made of single crystal silicon, and they are connected altogether to form a Wheatstone bridge. Since the silicon piezoresistors are having varied crystal orientation, variations of the piezoresistance coefficient would be generated, so that the performance of the device would be affected by the location and the orientation of the silicon piezoresistors on the Wheatstone bridge. As the circuit of the Wheatstone bridge is not balanced, the offset would not be approached to zero. The magnitude of the output signal would be affected by the impurity concentration and the temperature. The above-mentioned factors would increase the difficulties in forming the compensation circuit.
A membrane is fabricated by etching away the bulk silicon on a defined region until the required thickness is reached. Piezoresistors are integrated on the membrane, typically close to the edges. As the deformation and the stress are generated on the membrane by the stress, the resistance value would be varied by the silicon piezoresistor on the surface of the membrane, then the output voltage thereof can be magnified after sensed by the Wheatstone bridge. Wherein, as described above and shown in FIG. 1 (please refer to the reference material [2]), the piezoresistive coefficients would be changed according to the crystal orientation, so the efficiency of the device would be affected by the location and orientation factors of the silicon piezoresistors on the Wheatstone bridge. In addition, since the thermal expansion coefficient of the metal aluminum layer is different to that of the silicon nitride layer, the residual stresses would be generated during the process, and the suspended diaphragm so that the linearity for pressure sensing would be affected. Therefore, the interconnection of diaphragm region is mainly the interconnect with high doped concentration and low resistance.
As shown in FIG. 1, on a (100) silicon substrate, a maximum piezoresistance coefficient can be obtained by fabricating the silicon in the [110] direction, and a minimum piezoresistance coefficient can be obtained by fabricating the silicon piezoresistors in a direction set [100]. With respect to the present invention according to such relationships, the piezoresistors with high resistance are fabricated on the silicon suspended diaphragm towards the direction set [110] so that a maximum value of the resistance variation can be obtained; the interconnect with low resistance for connecting the piezoresistors on the diaphragm are fabricated towards the direction set [100] in order to reduce the influence upon the interconnect caused by the stresses. The resistance of the Wheatstone bridge is mainly contributed by the piezoresistors on the set of direction set [110], thus the linearity of the pressure-sensing device can be enhanced. The balance between the four resistors on the Wheatstone bridge is also an important consideration since the mismatch between the resistance value would cause the offset of the output voltage. In addition, the influence of the temperature can be reduced by designing the piezoresistance impurity concentration between 1019-1020 cm−3. Based on the above-mentioned design principle, the linearity of the output signals can be better, and the influence of temperature can be decreased.
3. Description of the Prior Art
There are some fabrication methods for the piezoresistive pressure sensor disclosed previously, and the brief descriptions are as below:                (1) In the U.S. Pat. No. 4,672,411 disclosed by Shimizu et al., a couple of p-type piezoresistors along the direction <110> are fabricated onto the membrane made of n type (100) silicon. Wherein a p+ type interconnect, which is highly doped and along the direction set [100], is a connection between the couple of the piezoresistors. Similarly, a p+ type interconnect, which is highly doped and along the direction set [100], is also fabricated to the another two ends of the two piezoresistors for connecting themselves with the metal interconnect out of the membrane. Please refer to FIG. 2 and the reference material [3] for details.        (2) In another investigation disclosed by S.-C. Kim et al. [4 ], a pressure sensor is fabricated onto a square membrane made of n type silicon, wherein a piezoresistor 320 with a strip is fabricated to each of the two opposite sides of the diaphragm, and a piezoresistor with two strips is fabricated to each of another two opposite sides, then the four resistors are connected by a metal interconnect 330 and a Wheatstone bridge is formed. Please refer to FIG. 3 and the reference material [4]. During the fabrication process, as the piezoresistors, which is parallel to the diaphragm, is getting nearer to the edge of the diaphragm, the larger stresses would be generated, so the piezoresistor with a single strip is applied in order to obtain a higher output signal. With respect to the piezoresistors of the vertical to diaphragm, as the piezoresistor is getting longer, the average stresses would be smaller. In order to maintain the symmetry and the balance of the Wheatstone bridge, the piezoresistor 320 is fabricated into two strips, and a higher resistance and output signal can be obtained.        
In addition, the piezoresistance coefficient can be expressed by its room-temperature value, referred to as π (300K), multiplied by a dimensionless factor that is a function of doping concentration (N) and temperature (T) (please refer to the reference material [5]):    π(N,T)=P(N,T) π(300K)    π(N,T): piezoresistance coefficient    P(N,T): piezoresistance factor    π(300K.): the piezoresistance coefficient under the room temperature (T=300K)
In the reference material [5], FIG. 4 shows the piezoresistance factor as a function of impurity concentration and temperature. The piezoresistance factor decrease with increasing impurity concentration. As the impurity concentration larger than 1019 cm−3 is selected for fabrication, an advantage that the piezoresistor is less influenced by the temperature is generated. Also, as to the consideration of lowering the fabrication cost of the devices, there is a tendency to reduce the size of the pressure sensor, so the dimension of the piezoresistors becomes longer relative to the sensing device and the linearity of the device and the magnitude of the output signals would be influenced, therefore, it must to reduced the length of the piezoresistors. Further, in order to decrease the low temperature effect, a highly impurity concentration is applied, thus the resistance value of the piezoresistor is reduced. In conclusion, as the length of the piezoresistor and the resistance of the piezoresistor diaphragm are becoming smaller, more strip piezoresistors should be applied in order to maintain a required resistance, thus it is enable to correspond with the electric specifications and applications.